The contract offers my mother received involved good money, but that was actually part of the problem. She got the impression that most of the managers at "Big Corp" had little/no regard for these "critical" systems; they were just looking to save their own jobs, either by way of a miracle worker (wave a magic wand and fix 40 years of tech debt in 3 months) or a fall guy (it's the contractor's fault, not mine).

Also, COBOL work is kinda crap. Debugging 20-year-old Java code sucks, but a 40-year-old COBOL project, with source code backed up to 5-inch floppy disks - forget about it. (I just watched the trailer for 'The Many Saints of Newark'. LOL)

It needs to be breathtaking money to be worth it when you take into account the risk of investing in a shrinking skill set

I think it depends. On my side, I'm OK with a modest salary if it's stable for the next 10-20 years. I think mainframe fits perfectly. It's declining but not that fast. It's unattractive so you face little competition. It's just a hell lot hard to get in in the middle of your life.

You can get that with a regular developer job with the threat of your job being a dead end.

Good point. Maybe new developer salary is different from what contractors get? The number I see for contractors are pretty good but yeah they have decades of experience under their belt.